In a reproduction process, a black-white scanner is used for the purpose of obtaining, from an image, a corrected reproduction which is adapted to the subsequent printing process and from which the printing form is later reproduced.
For the obtaining of an image signal, an image which is mounted in a stretched out manner on a rotating scanning drum, is scanned point-by-point and line-by-line. Depending on whether a normal image or a transparency image is to be reproduced, the light which depending on the image type is either reflected or transmitted by the image, arrives at a scanning member, which is guided parallel along the scanning drum, and is photo-electrically converted by the scanning member into the image signal. The image signal represents the brightness or tone-values of the scanned image points. The tone-value extent is delineated by the tone-value "white" and "black." Through the scanning of a white image point of the image pattern, the so-called white point or level of maximal brightness, a maximum image signal is created and by scanning an image point, of lowest brightness, sometimes referred to as black point, a minimum image signal is created.
The image signal arrives at a gradation stage, where a basic gradation is preselected which depends on the print type, the printing process and the print carrier, and where, in addition, the gradation for the desired editorial changes of the reproduction with respect to the original, is varied.
The image signal which is modified in the gradation stage, modulates the brightness of a recording lamp serving as recording member. The record-carrier which is in the form of a film, is mounted in a stretched-out manner on a similarly rotating recording drum and is illuminated point-by-point, and line-by-line by the recording lamp carried axially along the recording drum. The light exposed and developed film serves for the production of the printing form.
The brightness or tone-values "white" and "black" are fixed end-values of the gradation curve. For the change of the tone-value course of the reproduction with respect to the original, one solely changes the shape of the curve between the fixed end values. In order to establish these end values, standardize voltage levels of the image signal are assigned to the brightness or tone-values "white" and "black" which, in the following, will be designated as white-level for the brightness or tone-value "white" and as black-level for the brightness or tone-value "black." However, since the images have different brightnesses and contrasts, dissimilar image-signal voltages occur during the scanning of the of maximal or minimal brightness points of the different images. Therefore, one is faced with the task of having to adjust the image-signal voltages to the black-level or to the white-level, prior to the reproduction of an image.
In the case of the conventional scanners, the adjustment takes place by means of adjusting potentiometers. For this purpose, a white-point of the image is placed below the scanning element and the image-signal-voltage is raised to the white-level through rotation of a potentiometer and with the aid of a voltmeter. Subsequently, a black-point, or point of lowest brightness of the image is scanned and the corresponding image-signal-voltage is lowered to the black-level by changing the setting of an additional potentiometer.
These adjusting processes have to be controlled and, should the need arise, must be iteratively repeated, since an accurate adjustment can only take place through approximation, owing to the fact that the potentiometers are limited in their operational accuracy.
A color scanner is used for the reproduction of a colored image-pattern, in order to produce brightness corrected individual colors, or color components, from the image-pattern to be reproduced. The colors, or color components again serve for the production of printing forms of a set of color plates for multi-color printing.
In the case of a color scanner, the light which had been reflected or transmitted by the colored image-pattern is split up into three partial rays, which are conveyed to one color-channel each. The following are assigned to the color-channels, namely color filters for the color separation and photo-electrical transducers for obtaining of three color-signals, which represent the color portions of the scanned image points, whereby the color-signals which had been converted into logarithmic signals, are proportional to the color densities.
For the purpose of color-correction, the color-signals are conveyed to a color-computer, at the outputs of which three corrected color component-signals are available for the recording of the colors, or color-components "magenta," "cyan" and "yellow." Via a color-separation-switch, the color-separation-signals are successively selected and, via a gradation-stage, are conveyed to a recording lamp, the brightness of which is modulated by the selected color-component-signal. The recording of the colors or color-components takes place by a point-by-point and line-by-line light exposure of a film, by means of the recording lamp. The light-exposed and developed film is the desired component of the image.
In the case of a color scanner it will likewise be necessary -- prior to the recording of the color separations to set the white-level and the black-level individually for each image to be reproduced. In addition, an adjustment or balancing of all color-channels to the same sensitivity is necessary for the photo-electrical conversion of the incident scanning light into the color-signals.
For the adjustment of the color-signal-level of the white-point or level of maximum brightness, and of the black-point or level of lowest brightness, and for the balancing of the sensitivities of the color-channels, potentiometers are provided in the conventional color scanners, which potentiometers are likewise limited in their action and therefore require an iterative adjustment.
For the standardization of the color-signal-voltages two cases are to be distinguished:
a. The colored image pattern has a level of maximal brightness, and a level of lowest brightness, thus a "white-point" and a "black-point." In this case the color-signal-voltages of all color-channels which are obtained by the scanning of the "white-point," or point of maximal brightness have to be adjusted to the "white-level," and the color-signal voltages obtained by the scanning of the "black-point" or level of minimal brightness, have to be adjusted to the "black-level." Through the leveling of all color-signal-voltages to the white-level, a separate adjusting of the color-channels to the same sensitivity is not necessary.
To carry out the task described herein, the operating individual has to measure the color-signal-voltages separately for each color-channel with the aid of a voltmeter and carry out the balancing process through rotation of the potentiometer.
b. The colored image-pattern does not have a white or black image-point or levels of maximal and lowest brightness.
In this case, the brightest colored image point is selected as representing the "fictitious white-point" or maximal level, and the darkest image point is selected as the "fictitious black-point." The fictitious levels differ thereby from the true white-point or black-point by a color cast.
If this color cast is to be eliminated during the reproduction, that is to say if, for example, the darkest colored image-point is be reproduced as a purely black level, then the distinguishable color-signal-voltages obtained during the scanning of the fictitious black-point or level of lowest brightness have to be normalized to the black-level, and the color-signal voltages which had been obtained during the scanning of the fictitious white-point have to be normalized to the white-level. Thus, the task to be solved here is identical with the one described under point (a).
However, if this image is to be reproduced true to color, then the following procedure has to be carried out:
First of all, the color-channels need to be balanced to the same sensitivity and the color-signal-voltages, which have originated during the scanning of the fictitious white-point or level of maximal brightness, are adjusted to preset nominal or reference values.
Then the fictitious black-point or level of lowest brightness of the image is scanned while maintaining the previously adjusted values, whereby -- depending on the color-composition of the fictitious black-point -- three distinguishable color-signal-voltages are created, of which one may be at a minimum or minimal. value. This minimum color-signal-voltage is then lowered or reset to the black-level. However, for a color-true reproduction, the relationships of the three color-signal-voltages characterizing one image-point must be mutually retained, so that after adjustment of the minimum color-signal-voltage to the black-level, also the other two color-signal-voltages must be proportionally reduced by a correction-factor which correspond to the difference between the minimum value and the black-level.
For the adjustment or setting of the color-signal-level of the black-point, or level of lowest brightness, the operation has to measure first of all all color-signal-voltages with a voltmeter, then has to ascertain the minimum value and calculate the correction factor. With the aid of the calculated correction factor, the operator then has to determine the value to be set, and set the same by rotation of potentiometers. Since the potentiometers have their limitations, when balancing for the black-, and white-level, the adjustments performed have to be repeatedly controlled and corrected in cases of need, which involves a considerable expenditure in time.
However, for a rational use of a scanner it is necessary that the adjustment time for a reproduction process be of as short a duration as possible with respect to the actual reproduction time. However, shorter adjustment times can only be attained, if as many adjustments as possible are automatized.
Attempts have been made to replace the potentiometer which is required for the level-balancing and which is to be manually operated, by a motor-driven potentiometer. However, the use of this motor-driven potentiometer does not lead to the desired result, since -- owing to the required high degree of accuracy -- the attained adjustment time is not substantially shorter than in the case of the conventional method.